ABSTRACT Acquired and inherited diseases of myelin are the major cause of non-traumatic neurological disability in young adults in the USA. Studies have also described myelin pathology in brains from individuals with amyotrophic lateral sclerosis (ALS) and Alzheimer's disease (AD), and myelin protein gene alleles are risk factors for schizophrenia, depression, and autism. In addition to its insulating properties, therefore, myelin has multiple effects on neuronal function. It is now accepted that axonal and neuronal degeneration cause permanent neurological disability in individuals with primary myelin disease. My laboratory played a significant role in identifying axonal and neuronal degeneration in brains from individuals with multiple sclerosis (MS), an inflammatory demyelinating disease of the human central nervous system (CNS). We leveraged these data to develop animal models that recapitulate mechanistic aspects of myelin-induced axonal and neuronal degeneration. The purpose of the present proposal is to consolidate three NINDS R01s that investigate mechanisms of myelin-induced neurodegeneration. We will address three key questions. 1) How does myelin provide trophic support to axons? We propose that transfer of ATP substrates is the major mechanism by which myelin provides trophic support to axons. Disruption of this metabolic coupling in inherited myelin diseases induces mitochondrial pathology/degeneration in paranodal axoplasm, which causes axonal degeneration. 2) How does demyelination affect neurons and their synaptic connections? We propose that demyelination alters neuronal gene expression, modulates dendritic structure, and reduces neuronal viability; we further propose that remyelination will reverse these changes. 3) How does subpial cortical demyelination occur? We propose that subpial demyelination occurs by mechanisms that differ from immune-mediated mechanisms that demyelinate white matter and that novel therapies are needed to prevent subpial demyelination. The biggest challenge facing the myelin research community is the development of neuroprotective therapies. R35 funding will consolidate our efforts to identify new therapeutic targets that cause axonal and neuronal degeneration in myelin diseases. This is essential for the development of neuroprotective therapies that delay and possibly reverse permanent neurological disability in individuals with myelin disease.